Irons are majorly found in the core and the mantle, however, the crust is estimated to comprise about 5% iron.
Oxygen often thought of as atmospheric gas, is found mainly on the crust, and it is the second most abundant element on earth, it makes up roughly 46.6 percent of the crust and 30 percent of the earth's mass.
Silica compounds are mainly found in both the mantle and the crust, and it is said to be the third most abundant element, and it accounts for about 15% of the earth's mass.